Heretofore, as a positive active material for a lithium secondary battery, a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure has been examined, and lithium secondary batteries including LiCoO2 have been widely put to practical use. However, the discharge capacity of LiCoO2 is about 120 to 130 mAh/g. In addition, as a transition metal that forms the lithium transition metal composite oxide, it has been desired to use Mn that is abundant as an earth resource. However, there is the problem that when the molar ratio (Mn/Me) of Mn in the transition metal (Me) that forms the lithium transition metal composite oxide is more than 0.5, the structure is changed to a spinel type-structure when the battery is charged, and thus it is unable to maintain a crystal structure, resulting in very poor charge-discharge cycle performance.
In view of this, various materials which are excellent in charge-discharge cycle performance and in which the molar ratio (Mn/Me) of Mn in the transition metal that forms the lithium transition metal composite oxide is 0.5 or less have been proposed as positive active materials. For example, a positive active material containing LiNi2Mn1/2O2 or LiNi1/3Co1/3Mn1/3O2 has a discharge capacity of 150 to 180 mAh/g, and some of such positive active materials have been put to practical use.
The charge-discharge cycle performance of a battery including the above-mentioned lithium transition metal composite oxides as a positive active material is known to depend on the type and composition of an element that forms the transition metal, as well as the crystal structure of the active material, powder characteristics, a surface treatment and so on.
Non-Patent Document 1 suggests that a positive active material including LiNi1/3Co1/3Mn1/3O2 has a small unit volume change associated with a charge-discharge cycle.
In addition, inventions are known in which a full width at half maximum for a diffraction peak attributed to the (003) plane and the (104) plane of a positive active material containing a lithium transition metal composite oxide is specified (see Patent Documents 1 to 4).
Patent Document 1 discloses “a lithium secondary battery comprising a positive electrode and a negative electrode, the positive electrode including a positive active material of hollow structure which has a shell portion, and a hollow portion formed in the shell portion, the positive active material satisfying the following requirements: the positive active material contains a lithium transition metal oxide having a layered crystal structure, and the lithium transition metal oxide contains at least one metal element MT among Ni, Co and Mn; the ratio (A/B) of a full width at half maximum A for a diffraction peak given by the (003) plane and a full width at half maximum B for a diffraction peak given by the (104) plane is 0.7 or less a powder X-ray diffraction pattern using a CuKα ray; and the content of a compound containing Li and CO3 is 0.2% by mass or less” (claim 1).
Patent Document 2 discloses “a positive active material for a lithium secondary battery which comprises a lithium transition metal composite oxide represented by the composition formula: Li1+αMe1−αO2 (Me is a transition metal element including Co, Ni and Mn, and 1.2<(1+α)/(1−α)<1.6), wherein in the lithium transition metal composite oxide, the molar ratio (Co/Me) of Co to the Me is 0.24 to 0.36, and when the space group R3-m is used as a crystal structure model on the basis of an X-ray diffraction pattern, the full width at half maximum for the diffraction peak attributed to the (003) plane is in a range of 0.204° to 0.303°, or the full width at half maximum for the diffraction peak attributed to the (104) plane is in a range of 0.278° to 0.424°” (claim 1).
In addition, Patent Document 3 discloses “a positive active material comprising at least one secondary particle including an aggregate of two or more primary particles, wherein the secondary particle contains a nickel-based lithium transition metal oxide, the primary particle has an average particle size of 3 to 5 μm, the secondary particle includes at least one selected from a small-diameter secondary particle having an average particle size of 5 to 8 μm and a large-diameter secondary particle having an average particle size of 10 to 20 μm, and the full width at half maximum for the (003) peak is 0.120 to 0.125° in X-ray diffraction analysis spectroscopic analysis” (claim 1), and “the positive active material according to claim 1, wherein in X-ray diffraction analysis spectroscopic analysis, the full width at half maximum for the (104) peak is 0.105 to 0.110°, and the full width at half maximum for the (110) peak is 0.110 to 0.120° ” (claim 2).
Patent Document 4 suggests that examples of the positive active material containing a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure include Li1.02Mn0.45Ni0.45Co0.10O2 (C9), Li1.02Mn0.30Ni0.30Co0.40O2 (C10), Li1.02Mn0.40Ni0.50Co0.10O2 (C11), Li1.00Mn0.20Ni0.70Co0.10O2 (C13) and Li0.99Mn0.50Ni0.44Co0.06O2 (C14), which have full widths at half maximum for the diffraction peak of 0.142°, 0.125°, 0.134°, 0.134° and 0.130°, respectively, at 44.1±1° (see claims 7 and 8, paragraphs [0698] to [0701], [0719] to [0722], [0740] to [0743], [0782] to [0785], [0803] to [0806], and Tables 3 and 5).
In addition, Patent Document 4 suggests that “the positive active material according to claim 7 is characterized in that the full width at half maximum for the diffraction peak at 2θ:18.6±1° is 0.05° to 0.20°, and the full width at half maximum for the diffraction peak at 2θ:44.1±1° is 0.10° to 0.20°” (paragraph [0071]), and “with the configuration, there can be provided a positive active material making it possible to produce a nonaqueous electrolyte secondary battery having a high energy density (high discharge capacity) and excellent charge-discharge cycle performance” (paragraph [0072]).
Patent Document 5 suggests that a specific element is caused to exist on the surfaces of particles in a positive active material in the following text “[Example 13] Except that a titanium oxide powder and a lithium fluoride powder were further added in mixing of two compounds: cobalt hydroxide and lithium carbonate in Example 10, the same procedure as in Example 11 was carried out to synthesize a positive active material. The result of elemental analysis showed LiCo0.997Ti0.003O1.998F0.002. The particle size distribution of a powder with the above-described composition, which was formed by aggregation of primary particles obtained by crushing a fired product of the positive active material, was measured with water as a dispersion medium using a laser scattering particle size distribution measuring device, the result showed that average particle sizes D50, D10 and D90 were 13.2 μm, 10.1 μm and 16.3 μm, respectively, and a substantially spherical LiCoO2 powder having a specific surface area of 0.48 m2/g as determined by a BET method was obtained. For the powder, an X-ray diffraction spectrum was obtained using an X-ray diffraction apparatus (RINT 2100 Model manufactured by Rigaku Denki Co., Ltd.). In powder X-ray diffraction using a CuKα ray, the full width at half maximum for the diffraction peak of the (110) plane at 2 η=66.5±1° was 0.125°. When the powder was pressed at 0.3 t/cm2 by a hydraulic press machine, the apparent density after pressing was 3.26 g/cm3. The result of examination by spectroscopic analysis showed that titanium and fluorine were localized on the surface.” (paragraphs [0063] and [0064]).
Patent Document 6 also suggests that a specific element is caused to exist on the surfaces of particles in a positive active material in the following text “[Example 14] 196.72 g of cobalt oxyhydroxide having a Co content of 60.0% by mass and an average particle size of 13 μm, 74.71 g of lithium carbonate having a Li content of 18.7% by mass and an average particle size of 5.6 μm, 0.79 g of aluminum hydroxide having an Al content of 34.45% by mass, and 0.10 g of lithium fluoride were mixed in a mortar, and the resulting mixture was fired in air at 1000° C. for 10 hours to obtain a powder of a lithium cobalt-containing composite oxide. The composition of the resulting lithium cobalt-containing composite oxide was LiCo0.995Al0.005O1.998F0.002. The ratio Li/(Co+M) was 1.00. Subsequently, an aqueous solution of diammonium hydrogenphosphate was sprayed to the resulting lithium cobalt-containing composite oxide in such a manner that the concentration of phosphorus was 1 mol %, the mixture was then mixed, and the resulting mixture was heat-treated at 900° C. for 12 hours to obtain a positive active material. The resulting positive active material had an average particle size D50 of 14.3 μm and a specific surface area of 0.22 m2/g. For the resulting positive active material, an X-ray diffraction spectrum measured in the same manner as in Example 1 was analyzed using a powder X-ray diffraction method, and elemental analysis of a particle cross-section of the positive active material was performed using EPMA. As a result, it was able to confirm that the surfaces of particles of the lithium cobalt-containing composite oxide as a parent material had orthorhombic lithium phosphate having a composition of Li3PO4.” (paragraphs [0101] and [0102]).
Meanwhile, it is known that in production of a positive active material, ammonium fluoride or lithium fluoride is used for a lithium transition metal composite oxide (see Patent Documents 7 and 8).
According to Example 3 in Patent Document 7, “2 mol % LiNO3 was dissolved in 150 ml of distilled water in a 500 ml beaker, a commercially available LiCoO2 active material was carried therein, and the mixture was then stirred. Separately, 150 ml of a 2 mol % NH4F solution was continuously supplied to the solution at a flow rate of 1 ml/min while the temperature of a reactor was kept at 80° C., a coprecipitation reaction was carried out, and the mixture was then stirred for 24 hours. Here, the average temperature of the reactor was kept at 80° C. The reason why the temperature of the coprecipitation reaction is kept high as described above is that by coprecipitation of LiF, a precipitate having a high dispersion degree can be obtained in a complex salt state at a high temperature. The LiCoO2 active material coated with a fluorine compound was washed with distilled water, dried in a hot air thermostatic bath at 110° C. for 12 hours, and then heat-treated at 400° C. under an inert atmosphere to obtain final LiF coated LiCoO2. (paragraph [0048])”
According to Example 4 in Patent Document 8, “8 L of water was added in a closed reaction bath, and held at 45° C. while a nitrogen gas was circulated. Further, a mixed sulfate aqueous solution of Ni, Co and Mn and a sodium carbonate aqueous solution were continuously added with stirring in such a manner that the pH was 8.0 (±0.1). A reaction was carried out while during the reaction, only a filtrate was discharged to the outside of the system by a concentrator, and a solid component was retained in the reaction bath, and a coprecipitation product slurry was then collected. The collected slurry was filtered, washed with water, and dried at 100° C. overnight to obtain a coprecipitation precursor powder” (paragraph [0101]), and “The resulting coprecipitation precursor and the lithium carbonate powder were weighed, and thoroughly mixed. The mixture was fired for 10 hours at 870° C. under circulation of air in an electric furnace to obtain an intermediate fired product. 100 g of the intermediate fired product was added to 20 ml of a 0.95 mol/1 ammonium fluoride aqueous solution held at 30° C. with stirring. Next, 3 ml of a mixed aqueous solution adjusted so as to have a sulfuric acid concentration of 0.05 mol/1, an aluminum sulfate concentration of 1 mol/1 and a manganese sulfate concentration of 1 mol/l was added dropwise to the intermediate fired product slurry, filtered, washed with water, and dried at 90° C. The resulting product was fired at 450° C. for 3 hours under circulation of air in an electric furnace to obtain a positive active material particle powder” (paragraph [0102]).
In addition, it is also known that in production of a positive active material as described above, a salt of a strong acid such as lithium nitrate or lithium fluoride is used (see, for example, paragraphs [0117] to [0118] in Patent Document 8 and claim 7 in Patent Document 9).
It is also known that when LiF is mixed as a sintering aid at the time of sintering a transition metal hydroxide coprecipitation precursor and lithium hydroxide in a lithium transition metal composite oxide synthesizing step, the density is increased, and structure stability associated with charge-discharge is attained (see, for example, Non-Patent Documents 2 to 4).
Further, regarding the porosity of a positive active material, Patent Document 10 discloses a positive electrode material in which a plurality of primary particles are aggregated to form secondary particles, the secondary particles has a porosity of 2.5 to 35%, and the secondary particles include crystals having a layer structure of a composite oxide represented by LiaMnxNiyCozO2, where 1≤a≤1.2, 0≤x≤0.65, 0.35≤y<0.5, 0≤z≤0.65 and x+y+z=1 (claims 3 and 5).
In addition, according to Patent Document 10, “FIG. 3 is a diagram showing a relationship between the porosity and the discharge capacity. When the porosity is 2.5% or less, the discharge capacity at room temperature is as low as 100 mAh/g, and on the other hand, when the porosity is more than 35%, the discharge capacity at a low temperature of −30° C. is sharply reduced. However, when the porosity is 2.5 to 35% as in the present invention, a high discharge capacity of about 150 mAh/g at 25° C. and 10 mAh/g or more at −30° C. can be obtained” (paragraph [0029]). Example 1 in Patent Document 10 suggests that a positive electrode material in which “the ratios Ni:Mn:Co and Li:(NiMnCo) are 1:1:1 and 1.02:1 in terms of an atomic ratio” (paragraph [0022]) is 3.6% (see Table 2).